This invention relates to a radial tire improved in the stability during straight travelling through reduction in ply steer.
The radial tire for passenger cars was conventionally formed by interposing at least two belt layers between a tread and a carcass layer in the direction substantially parallel to the circumferential direction of the tire. The reinforcing cords constituting one of the belt layer are provided at an angle of 15.degree. to 30.degree. to the circumferential direction of the tire, while the reinforcing cords constituting the other layer are provided at an angle of 15.degree. to 30.degree. to the circumferential direction of the tire and cross the above-mentioned cords. Meanwhile the carcass plies comprises one or two layers. The cords constituting each layer of the carcass plies are provided at about 90.degree. to the circumferential direction of the tire. The radial tire of this kind is superior in braking performance, fuel consumption, abrasion resistance, etc. to bias tires by virtue of the presence of the above-mentioned reinforcing belt layers. However, the radial tire has a problem that the stability during straight travelling is inferior. Specifically, when a radial tire is rotated and travelled, a lateral force is generated in either the left or the right direction with respect to the direction of travelling even in the case of a slip angle of zero degree. This lateral force often brings about the travelling of a vehicle in a direction different from that which the driver intends.
In general, the lateral force in the case of a slip angle of zero degree comprises two force components which occur through different mechanisms. One of the force components is called conicity (CT) while the other is called ply steer (PS), and the lateral force is categorized as part of the uniformity characteristics of a tire. In accordance with the testing method on uniformity of tires for automobiles (JASO C607), assuming that the average value of the lateral force required for making one revolution of a tire is LFD, the relationships represented by the following equations are established between LFDw which is a value as determined on one side of a tire, LFDs which is a value as determined on the other side of the tire after turn the tire inside out and the above-mentioned conicity (CT) and ply steer (PS): EQU LFDw=PS+CT (1) EQU LFDs=PS-CT (2)
From the above equations (1) and (2), PS and CT are given as follows: EQU CT=(LFDw-LFDs)/2 (3) EQU PS=(LFDw+LFDs)/2 (4)
Meanwhile it is believed that among the above-mentioned conicity and ply steer the conicity is a force which is generated when a tire having a geometrically asymmetrical shape as viewed in the circumferential direction of the tire, i.e., having a truncated cone shape, is rolled. The generation of this force is attributable mainly to the influence of the location of the belt layers which are inserted into the tread and the carcass layer of the tire. Therefore, it is possible to reduce the conicity through any improvement in the production of the tire. On the other hand, the ply steer is an inherent force attributable to the structure of the belt layers. Therefore, it is substantially impossible to reduce the ply steer unless the structure of the belt layers is changed.